When the Supreme Court deregulated independent political spending four years ago, the court reasoned that unrestricted money posed no corruption risk because a firewall separates candidates from their outside benefactors.

As Justice Anthony M. Kennedy wrote for the majority in Citizens United v. Federal Election Commission: “By definition, an independent expenditure is political speech presented to the electorate that is not in coordination with a candidate.” Such expenditures, the court concluded, “including those made by corporations, do not give rise to corruption or the appearance of corruption.”

Four years after that ruling, the supposed barrier between candidates and unrestricted super PACs is flimsier than ever. As midterm elections approach, complaints are rolling into the FEC from both parties about super PACs that share vendors, fundraisers and video footage with the politicians they support.

Not that anyone expects much response from the FEC. The agency has been fighting in court for years to defend its definition of illegal coordination, which watchdogs allege is too narrow and contradicts campaign finance law. Indeed, FEC rules explicitly permit quite an array of candidate-super-PAC interactions.

Politicians may raise money for super PACs and even appear at their events, for example, as long as they never ask for checks larger than the amounts donors may write directly to their campaigns — $5,000 for a political action committee and $2,600 for an individual per election.

Super PACs launched by the close advisers and top aides of the candidates they end up backing do not necessarily run afoul of the law. Nor does the candidate’s sharing of consultants, fundraisers or media buyers with the PAC.

Even under these anything-goes rules, however, politicians and their backers are inviting fresh coordination allegations. Last month the Arizona Republican Party complained to the FEC that an ad paid for by House Majority PAC violated coordination rules because the Democratic super PAC had captured and broadcast video footage first posted by Rep. Ann Kirkpatrick, D-Ariz.

The state GOP accused Democrats of “stepping over the legal line to try to sway voters” for Kirkpatrick. A House Majority PAC spokesman called the complaint “absolutely, 100 percent without merit” and noted that the FEC closed the books without action on two previous complaints that ran along the same lines.

Several other Democrats have posted conveniently placed video footage on their websites, including House member and Senate candidate Bruce Braley of Iowa and Sens. Kay Hagan of North Carolina and Mark Begich of Alaska. Last year, several House Democrats sang the praises of House Majority PAC in a video testimonial aimed at donors.

Sen. Mark Pryor, D-Ark., recently created a website that features scripted attacks on his GOP challenger, Rep. Tom Cotton, some of which resurfaced close to verbatim in an anti-Cotton ad run by the Democrat-friendly outside group Patriot Majority USA.

In the meantime, Democrats in Michigan have called on the FEC to investigate GOP Senate candidate Terri Lynn Land’s apparent acknowledgment that her campaign had communicated with super PACs. Land reportedly said at a public forum that her campaign “had talked to a lot of those folks. They’re committed to Michigan.” A Land aide has denied any coordination.

Sharing video footage between candidates and super PACs may not violate the FEC’s coordination rules, said Paul Ryan, senior counsel at the Campaign Legal Center, but it runs afoul of another FEC law — one that treats dissemination of campaign materials as an in-kind political contribution. Such contributions are illegal for super PACs, Ryan noted.

“I think that political players are likely emboldened by the lack of enforcement action by the FEC in recent years,” said Ryan. The super PAC American Crossroads explicitly asked the FEC in 2011 for permission to work directly with candidates to produce issue ads, but the agency deadlocked and took no action.

The Campaign Legal Center has yet to hear back from the FEC following its complaint in 2012 that the super PAC backing Mitt Romney illegally coordinated with the GOP nominee when it rebroadcast an entire ad produced by the Romney campaign in 2007, during his previous presidential run.

The challenge for those lodging coordination complaints, said former Republican FEC Chairman Bradley Smith, is that they are tough to prove unless super PAC organizers and candidates engage in direct or face-to-face communications — the kind that could facilitate quid pro quo corruption. Telling super PACs they can’t pick up b-roll footage from candidate websites is both questionable and futile, he argued.

“I just don’t see how you are going to realistically tell people that you can’t use material that’s out there that everybody knows about,” said Smith, currently the chairman of the pro-deregulation Center for Competitive Politics. “It creates inherent line-drawing problems.”

Eliza Newlin Carney is a senior staff writer covering political money and election law for CQ Roll Call.